This is a very short, and “simple” explanation how i made Spotify 0.4 play local MP3 files under Linux with Wine.
I have not tested this elsewhere, and i can not guarantee that everyone has to do the same steps as me.

If you want to try my finished file you can download it here: winemp3.acm.so (x86 only)
Place it in /usr/lib/wine/ (x86) or /usr/lib32/wine/ (x64).

Or if you need to do things a tad more automatic you can try this script which will do everything automatically for the ones of you on x86: winefix.sh

For those of you who want the technical details: continue reading 🙂

You should try and skip step 3 if possible, but if Spotify just hangs and max out your CPU you should do step 3.

You can also use an hex editor to change WINE-MPEG3 to LAME-MPEG3 in /usr/lib/wine/winemp3.acm.so to try before doing the, for me, crucial step number three.